Conventional microelectronic devices can include a substrate having a front side on which semiconductor elements and other features are formed and a back side opposite the front side. One conventional technique for forming semiconductor elements and other features on the substrate is to etch selected portions of the front side of the substrate and apply a succession of conductor, semiconductor and/or insulator layers to the substrate which together form the elements. In a typical operation, a layer of photosensitive etch-resistant material (resist) is applied to the center of the substrate and the substrate is spun to distribute the resist over the substrate by centrifugal force. Selected portions of the resist are then exposed to a selected radiation while a mask covers the unselected portions. The radiation causes the selected portions to become soluble (in the case of a positive resist process) or insoluble (in the case of a negative resist process) when exposed to a selected solvent. The solvent washes away the soluble portion of the resist, leaving the remaining portion of the resist to cover selected portions of the substrate. The exposed portions of the substrate are then etched away and the remaining resist is removed to leave one portion of the substrate recessed relative to the surrounding portions. The recessed area can be filled (or the adjacent elevated area can be built up) with the succession of conductor, semiconductor and/or insulator layers to form the semiconductor elements.
During the process discussed above, the resist is exposed to a critical dose of radiation that causes the selected or exposed portion of the resist to become either soluble or insoluble. Typically, the entire resist layer is exposed to the same critical dose of radiation. However, if the thickness of the resist layer is not uniform, the critical dose of radiation may not provide the appropriate exposure level. For example, if the resist layer is locally thick, it may be underexposed, and may not completely change its solubility. If the resist layer is locally thin, it may be overexposed and reflections from the overexposed regions may strike adjacent regions, potentially altering the geometry of the features formed on the substrate, or causing the features to overlap. Accordingly, it is important to maintain the resist layer at a uniform thickness so that a single critical dose of radiation will have the same effect on the entire resist layer.
One factor that controls the thickness of the resist layer is the temperature of the semiconductor substrate on which the resist layer is disposed. For example, where the temperature of the substrate is elevated, solvents in the resist will evaporate more rapidly, thickening the resist before it spins off the edge of the substrate, and causing a local increase in resist thickness. Conversely, where the temperature of the substrate is depressed, solvents in the resist are less likely to evaporate before the resist spins off the substrate, resulting in a local reduction in resist thickness.
To make semiconductor devices more compact, the size of the semiconductor elements on the devices are made as small as possible and are positioned as closely together as possible. Accordingly, it becomes increasingly important to control the thickness of the resist layer to ensure that the selected portions of the resist layer are exposed to the proper radiation dosage so that adjacent features are well defined and do not overlap.
One approach to controlling the distribution of the resist has been to cool the outer edge of the substrate by directing flow of rinsing solution toward the outer periphery of the back side of the substrate. The rinsing solution can cool the wafer as the rinsing solution evaporates, and can prevent the resist from flowing from the front side of the substrate around the outer edge of the substrate to the back side of the substrate. A drawback with this method is that it may not adequately control the temperature of the entire substrate, and may therefore fail to produce a uniformly thick layer of resist on the substrate. For example, even a 0.5.degree. C. change in the substrate temperature can have a large effect on the thickness of the resist layer, and this effect may not be adequately addressed by directing rinsing solution toward the outer periphery of the substrate.